Many paper and paperboard products are made with a multi-ply construction. Typically, the least contaminated fiber is utilized on the top side of the sheet as the printing surface, and more contaminated fiber is used in the central or outer layers.
The typical multi-ply mill has a very complex system to produce the individual plies that are used in producing paperboard. These systems include individual thick stock pumps and flow controls for the pulp of each ply, separate stuff boxes to maintain the desired pressure to separate basis weight valves, separate cleaner pumps and fan pumps, and often individual headboxes and forming fabrics for each ply. In addition, the system for each ply must be precisely controlled and coordinated with the other ply systems.
In producing the multi-ply product, it has been proposed to deliver the pulp stock from each ply system to a multi-channel headbox. One of the difficulties in this approach is that each channel of the head box is separated from other channels by a very thin membrane, usually composed of stainless steel. As the pressures in each ply system are not equal, the pressure differential in adjacent channels of the headbox can cause damage or deflection to the thin membranes.
Recycled fiber, such as old corrugated containers, newsprint, white office waste, and the like, is playing an ever expanding role in paperboard manufacturing. However, recycled fiber contains heavy contaminants, such as sand, dirt, bark, large fiber bundles called shieves, metal fragments, and the like, as well as lightweight "stickie" contaminants, which are predominantly hot melt adhesives, latexes, pressure sensitive adhesives, waxes and the like.
When using recycled fiber in producing multi-ply paper products, it is important that the heavy contaminants be isolated from the printing surface for the heavy contaminants can mar the printing surface by producing dark colored specks. Similarly, the "stickies" should be maintained out of contact with the clothing and drying surfaces of the papermaking machine, because the "stickies" may adhere to these components.